Hi, I am presently having a house built with Electric underfloor heating. The floor screed has been pumped/poured and seems to be calcium sulphate type. I have read varying reports as to the requirements for movement/expansion joints in the screed and tiles. The largest area in the house is 36m2 and there is going to be movements joints in the tles here and across all doorways and were separately controlled heating mats exist. However, the screed has been carried out without any such joints but I have read there is very little movement with calcium sulphate.
Are movement joints required in the screed ? How does this affect the tiles and those movement joints.
Is there rule of thumb scale answer. Any advice or guidance or expirence appreciated.
Thank you.
dingers
All heated screeds require movement control joints. Calcium sulphate screeds are no exception althought hey are very forgiving when joints are not placed due to the dimensional stability of the material. BS8204 part 7 is the over riding standard covering pumpable self smoothing screeds which includes those based on calcium sulphate. Generally the maximum bay size will be 300m2 but this will need to be jointed in accordance with the rules. outlined in the standard.
Section 6.1 which is the relevent section is quoted below
All joints in the screed should be reflected through the tiled surface. The paragraph at the end of the first section is appropriate.
6.10.1 Bay sizes, position of joints
Pumpable self-smoothing screeds are generally laid in large areas, often without the need for joints.
However, where there are structural movement joints or expansion joints in the base slab, it is essential
that these should be continued through the screed to the surface of the finished flooring, directly over the
joint in the base.
Where construction joints in the base slab have opened, or could open, they could reflect through an
overlying bonded screed. Similarly, at positions of rigid supports in base constructions which are subject to
deflection (e.g. supports to precast planks) there will be a risk of reflective cracking through an overlying
screed. In both cases, if the floor finish is to be bonded and rigid, or if the screed is to be a wearing screed,
a joint capable of accommodating the expected movement should be formed through the screed and (where
relevant) the floor finish.
For unbonded or floating screed constructions, large area pours (for example with an unbroken length
exceeding about 40m) might need stress relief joints at intervals. The proprietary screed manufacturers
advice should be sought. Long thin strips of screed are more liable to stress cracking, and areas such as
long corridors can benefit from joints at intervals to control such cracking.
Heated screeds should include joints across doorways and between areas with separate heating control
zones. If the underfloor heating manufacturer recommends additional joints, their advice should be
followed.
6.10.2 Construction of joints
Where a day joint is formed to divide up an area of work, but is not required to accommodate movement,
the joint should be formed to provide neat vertical edges to the poured areas of screed.
Where a stress relief joint is required, the joint should be formed to provide neat vertical edges to the
screed. Alternatively, joints may be saw cut into the completed and hardened screed, which procedure
should be carried out within a few days of application of the screed before any random cracking develops
elsewhere.
NOTE Structural movement and structural expansion joints are usually formed by incorporating a proprietary metal extrusion
movement joint within the zone of the screed and floor finishes.
6.10.3 Edge separation joints
Unbonded and floating screeds should be separated from all walls, columns and other upstands by a strip
of compressible foam material. The minimum thickness of foam should be 5mm, with thicker foam
recommended in large area pours, typically in areas exceeding about 40m in length, when the advice of the
proprietary manufacturer should be sought.
rrr:
